This invention relates to an ignition system for an internal combustion engine, and more particularly to an ignition system capable of causing a high voltage to be induced across a secondary winding of an ignition coil when a transistor switch means which is connected across a primary winding of the ignition coil is made non-conductive at an ignition position of an engine.
A prior art ignition system of such type is generally constructed in such a manner as shown in FIG. 1. In FIG. 1, reference numeral 10 designates an ignition coil including a primary winding 11 and a secondary winding 12; reference numeral 14 indicates an ignition plug attached through the both ends of the secondary winding 12 to a cylinder of an engine; and reference numeral 16 designates a npn-transistor forming a transistor switch means. A collector of the transistor 16 is connected to one end of the primary winding 11 of the ignition coil 10, and an emitter thereof is connected to the other end of the primary winding 11 through a diode 18, the diode 18 having an anode connected to the emitter. The transistor 16 is also connected through a base thereof to one end of a resistor 20, and a resistor 22 is connected between the other end of the resistor 20 and the collector of the transistor 16. The resistors 20 and 22 are connected to an anode of a thyristor 24 through a coupling point therebetween, and a cathode of the thyristor 24 is connected to a cathode of the diode 18. A resistor 26 is connected across a gate of the thyristor 24 and the cathode thereof, and a signal generating coil 28 is also coupled across the gate and cathode of the thyristor 24.
In the prior art ignition system as shown in FIG. 1, the ignition coil is conventionally disposed on a stator of an AC magnetic generator operated by an internal combustion engine. That is, the primary winding 11 also serves as an igniting exciter coil, so that an AC voltage may be induced across the primary winding 11 in synchronism with the rotation of the engine. The signal generating coil 28 is disposed in a signal coil which generates a signal by utilizing magnetic flux from the magnetic poles of a rotator of the magneto generator or in a signal generator provided separately from the magneto generator, and the coil 28 acts to generate a turn-on signal for making the thyristor 24 conductive at an ignition position of the engine. When a half-cycle of voltage is induced across the primary winding 11 in the direction of the arrow along the solid line of FIG. 1 correspondingly to the rotation of an output shaft of the engine, a base current flows from the primary winding 11 through the resistors 22 and 20 to the base of the transistor 16; so that the transistor 16 is made conductive to permit a current to flow from the primary winding 11 through a collector-emitter circuit of the transistor 16 and the diode 18. Then, when an output voltage of the signal generating coil 28 reaches a trigger level of the thyristor 24 at an ignition position of the engine, the thyristor 24 becomes conductive to prevent a current from flowing to the base of the transistor 16 and to cause the current to be by-passed through the thyristor 24. Thus, a base current to the base of the transistor 16 is interrupted. This causes the transistor 16 to be made non-conductive, so that a current flowing across the primary winding 11 is suddenly decreased, to thereby cause the change of magnetic flux across the primary winding. Such change of magnetic flux allows a high voltage to be induced across the secondary coil 12, so that the ignition plug 14 generates a spark between electrodes thereof to permit the engine to be ignited.
In the prior art ignition system constructed as mentioned above, an impedance between the collector and emitter of the transistor 16 decreases when the transistor 16 is conductive, so that the voltage across the primary winding 11 decreases to 2 to 3 V. This means that the resistor 22 should have a sufficiently low resistance to flow a base current enough to keep the transistor conductive. Therefore, when the thyristor 24 and the transistor 16 are made conductive and non-conductive respectively to induce a voltage across the primary winding 11 in the direction of the arrow along the solid line of FIG. 1, the voltage causes a considerable amount of current to flow through the resistor 22 and the thyristor 24; as a result thereof, the ignition performance of the system is adversely affected. The reason is that a secondary voltage induced across the secondary coil 12 and a spark energy are determined by a ratio of a change of the primary current (di) flowing through the primary winding 11 to a time (dt) "dt/dt" and by the amount of change of the primary current. Therefore, in the prior art ignition system, to induce a sufficiently high voltage across the secondary winding 12 and to supply a high spark energy, it is to be desired that the primary current flowing through the primary winding 11 rapidly decreases to zero at a transient time when the transistor 16 becomes non-conductive. However, since a large current flows through a circuit consisting of the resistor 22 and the thyristor 24 at a transient time when the transistor 16 is made non-conductive, the primary current does not decrease to zero; the primary current decreases to a value of a current determined by an impedance of the circuit consisting of the resistor 22 and the thyristor 24. As the result thereof, since the amount of change of the primary current and the di/dt decrease, both the spark energy and the secondary voltage also decrease so that the ignition performance is adversely affected. In addition, such prior art ignition system has another disadvantage that the transistor 16 is difficult to be operated in a low power loss region because it is impossible to flow a larger base current through the base of the transistor 16. In general, in order to operate a transistor in a low power loss region with a relatively small base current, the transistor is required to have a very large amplification factor. While, a transistor to be used in such ignition system is required to have characteristics of withstanding a high voltage induced across the primary winding 11 at the interruption of a primary current. In this connection, because a transistor of a high withstand voltage generally has a low amplification factor, it is very difficult to obtain a transistor having a high withstand voltage as well as a high amplification factor, thus, an ignition system including such transistor has a disadvantage to become expensive.